Studies of the transmural distribution of myocardial blood flow will be performed in chronically instrumented awake dogs using 15 micron diameter radionuclide labeled microspheres injected into the left atrium. To dissociate the effects of myocardial tissue pressure from active coronary vasomotion on the distribution of blood flow during cardiac systole, transmural myocardial perfusion will be examined when arterial inflow is limited to the interval of systole both during normal conditions and in the presence of maximal coronary vasodilation produced by intravenous administration of adenosine. Because of the suggestion that calcium antagonists may be of therapeutic benefit in patients with coronary artery disease, studies will be carried out to examine the effects of two such drugs, nefidipine and diltiazem, on regional myocardial blood flow in dogs with flow-limiting coronary artery stenoses. To test the hypothesis that right ventricular failure occurring in the setting of acute pulmonary artery hypertension results from right ventricular ischemia occurring after maximal vasodilation of the right coronary arterial system has been achieved, a study will be carried out to determine whether maximal coronary vasodilation does in fact occur prior to the onset of right ventricular failure in dogs subjected to acute pulmonary artery constriction.